ribbit scheme虚拟机

基于stack虚拟机
内存包括4部分：the Scheme objects, the Scheme global variables, the RVM’s stack, and the instruction stream executed by the RVM.
使用rib来表示object， [ x,x,x ]，整数用低位为1的word（host c）： an object reference is implemented with a machine word with the lower bit encoding the type: 1 when the object is an integer (the other bits represent the integer value), and 0 when the object is a rib (all the bits are the aligned address of the rib in memory).
虚拟机只需要两个指针：stack, which refers to the rib at the top-of-stack (TOS), and pc, which refers to the rib containing the RVM instruction being executed. （同时亦作为gc root）
使用 Cheney-style stop-and-copy GC 算法
类型：[ field0, field1, type ] 0 pair, 1 procedure, 2 symbol/global variable, 3 string, 4 vector, 5 special